PROJECT SUMMARY This project responds directly to PAR-17-203 ?Inter-organelle Communication in Cancer.? A new pathway was discovered called the ?mtDNA-IRDS? pathway, through which mitochondria send a stress signal to the nucleus to regulate expression of interferon-stimulated genes (ISGs), which are usually associated with antiviral immunity. The mtDNA-IRDS pathway was elucidated during investigations of a unique mouse model of mtDNA stress caused by haploinsufficiency of the nucleus-encoded mtDNA-packaging factor TFAM (i.e. Tfam+/- mice). These mice have altered mitochondria and nucleoid structures (the mitochondrial version of chromatin) that promotes release of mtDNA into the cytoplasm. The surprising feature of these mice (and cells isolated from them) is that this released mtDNA promotes innate immune signaling, resulting in upregulation of a unique subset of ISGs known as the Interferon-Related DNA-damage resistance Signature (IRDS). The IRDS promotes chemotherapy resistance of tumor cells. The Tfam+/- condition enhances spontaneous intestinal tumorigenesis In the APCMin/+ mouse model of human colorectal cancer (CRC) and new preliminary results show that knock-out of the tumor suppressor commonly mutated in CRC, ARID1A (a subunit of the nuclear SWI/SNF chromatin remodeling complex) also induces the mtDNA-IRDS pathway. Thus, the main premise of this proposal is that the mtDNA-IRDS pathway increases tumorigenesis and/or chemoresistance in CRC, which will be tested through completion of the following three Specific Aims. Aim 1 is to understand how mutations in ARID1A lead to mtDNA-IRDS pathway induction. Aim 2 is to elucidate the mechanism through which the mtDNA-IRDS pathway enhances nuclear DNA repair using the Tfam+/- model of mtDNA-mediated, mitochondria-to-nucleus signaling. Aim 3 is to investigate the role of the mtDNA-IRDS pathway in tumorigenesis and/or chemoresistance in vivo using the Tfam+/-, APCMin/+ and intestinal Arid1a knock-out mouse models of CRC and human CRC tumor samples. That the PARP9-DTX3L complex is a major driver of tumorigenesis and chemoresistance in vivo will be tested by crossing these CRC models to Parp9 knock-out mice. CRC is a leading cause of cancer-related deaths in both men and women, and treatment of this disease is plagued by resistance to chemotherapy. The significance of this proposal is that it probes a novel mechanism of tumorigenesis and chemoresistance due to mtDNA-IRDS pathway activation that has the potential for therapeutic targeting.